EventLogAppender: Add support for setting the Category on Event Log messages.

Details

Description

The EventLog supports Categories for each message. A category is a short integer.

This could either be implemented as a fixed category value defined on the appender at config time, or as a value read from the LoggingEvent properties. The second option is more flexible as it allows the category to be specified for each message, a default category could still be specified in the appender configuration to allow for scenario 1 as well.